Currently, typical power semiconductor transistors, including devices such as power MOSFETs and insulated gate bipolar transistors (IGBTs), are fabricated with silicon (Si) semiconductor material. More recently, silicon carbide (SiC) power devices have been considered due to their superior properties. III-Nitride or III-N semiconductor devices, such as gallium nitride (GaN) devices, are now emerging as attractive candidates to carry large currents, support high voltages and to provide very low on-resistance and fast switching times.
Typical III-N high electron mobility transistors (HEMTs) and related devices are formed on III-Nitride materials grown in a group-III polar orientation, such as the [0 0 0 1] (C-plane) orientation. That is, the source, gate, and drain electrodes of the HEMT are formed over the group-III face (e.g., [0 0 0 1] face) of the III-N material layers, which is typically on an opposite side of the III-N material layers from the substrate on which the III-N layers are formed. Furthermore, III-N HEMTs are typically normally-on devices, which means that they conduct current when zero voltage is applied to the gate relative to the source. These normally on devices are known as depletion mode (D-mode) devices. However, it is more desirable in power electronics to have normally-off devices, called enhancement mode (E-mode) devices, that do not conduct substantial current at zero gate voltage and require a sufficiently positive voltage applied to the gate relative to the source in order to turn on. The use of E-mode devices in power electronics can help reduce the potential for damage to the device or to other circuit components by preventing accidental turn on of the device in case of circuit failure.